1. Field of the Invention
The present invention relates to a power semiconductor device and a method for making the same, and more particularly, to an insulated gate bipolar transistor (IGBT) having a trench-type insulated gate structure and a method for making the same.
2. Description of the Related Art
An IGBT, an insulated-gate type power semiconductor device has both the high-speed switching characteristics of a power MOSFET and the high-output characteristics of a BJT. Thus, recently, the IGBT has been increasingly used in power electronic devices such as inverters, converters, switching power suppliers, etc. In particular, a trench IGBT avoids the resistive component generated by the parasitic junction-type FET effect, which has been an obstacle to improved characteristics of planar IGBTs, because it has the small onresistance of a bipolar device.
FIG. 1 is a cross-sectional view of a conventional trench IGBT.
Referring to FIG. 1, an n-type high-concentration buffer layer 2 and an n-type low-concentration drift region 3 are sequentially formed on a p-type high-concentration semiconductor substrate 1 used as a collector area. A p-type base region 4 is formed on the upper surface of drift region 3, and an n-type high-concentration emitter region 5 is formed on part of the upper surface of the base region 4. A trench 6 is formed through the emitter region 5 and the base region 4, on part of drift region 3. A thin gate insulative layer 7 is formed on the inner wall of trench 6. A gate electrode 8, i.e., an impurity-doped polysilicon layer, is formed on gate insulative layer 7. Meanwhile, an emitter electrode 9 is formed so as to be electrically connected to base region 4 and emitter region 5. A collector electrode 10 is formed so as to be electrically connected to semiconductor substrate 1.
In this trench IGBT, a channel region is formed on the side wall of trench 6 within base region 4. That is, if a forward bias is applied to gate electrode 8, the conductive type of the side wall of trench 6 within base region 4 is inverted to form a channel. An electron current flows from the emitter region 5 to the drift region 3 through this channel. The electron current acts as a base current of a pnp transistor formed by base region 4, drift region 3, and semiconductor substrate 1. Corresponding to the electron current, a hole current flows from semiconductor substrate 1 to emitter electrode 9 via drift region 3 and base region 4.
However, the most significant problem of such a trench IGBT is a latch-up phenomenon. That is, a parasitic pnpn thyristor structure exists even in the trench IGBT, this pnpn structure being comprised of the emitter region 5, the base region 4, the drift region 3, and the semiconductor substrate 1. A voltage drop occurs in the lower portion of the emitter region 5 due to the hole current (indicated by arrows) flowing from the semiconductor substrate 1 to the base region 4 through the drift region 3. Thus, if the voltage difference between the base region 4 and the emitter region 5 increases to the extent (e.g., 0.7V) of conducting a pn junction (J.sub.1), the parasitic pnpn thyristor operates to cause a latch-up. When latch-up occurs, the device can no longer be controlled by a gate voltage, and the device may become destroyed due to the flow of excessive current.